Genes and Protein Synthesis Chapter 7

One Gene-One Polypeptide Hypothesis DNA contains all of our hereditary information Genes are located in our DNA ~25,000 genes in our DNA (46 chromosomes) Each Gene codes for a specific polypeptide

Main Idea Central Dogma Francis Crick (1956)

Overall Process Transcription Translation DNA to RNA DNA molecule Gene 1 Gene 2 Gene 3 DNA strand TRANSCRIPTION RNA Polypeptide TRANSLATION Codon Amino acid Transcription DNA to RNA Takes place inside nucleus Translation Assembly of amino acids into polypeptide Takes place in cytoplasm

Key Terms RNA transcription TATA box Introns, Exons mRNA, tRNA, rRNA Initiation, Elongation, Termination TATA box Introns, Exons mRNA, tRNA, rRNA Translation Ribosome Codon Amino Acids Polypeptide

DNA RNA Double stranded Single stranded Adenine pairs with Thymine Adenine pairs with Uracil (A with U) Guanine pairs with Cytosine Guanine pairs with Cytosine (C with G) Deoxyribose sugar Ribose sugar Remember!! When transcribing DNA to RNA, T from DNA pairs with A for RNA and A from DNA pairs with U for RNA

DNA to Protein Protein is made of amino acid sequences 20 amino acids DNA molecule Gene 1 Gene 2 Gene 3 DNA strand TRANSCRIPTION RNA Polypeptide TRANSLATION Codon Amino acid Protein is made of amino acid sequences 20 amino acids How does DNA code for amino acid?

Genetic Code Codon AA are represented by more than one codon Three letter code 5’ to 3’ order Start codon (AUG/methionine) 3 Stop codons AA are represented by more than one codon 61 codons that specify AA 4³ = 64 minus 3 stop codons = 61

Amino acids

Transcription DNA to RNA Occurs in nucleus Three process Initiation RNA polymerase DNA of gene DNA to RNA Occurs in nucleus Three process Initiation Elongation Termination Promoter DNA Terminator DNA Initiation Elongation Termination Growing RNA Completed RNA RNA polymerase

Initiation RNA polymerase binds to DNA Binds at promoter region TATA box RNA polymerase unwinds DNA Transcription unit Part of gene that is transcribed Transcription factors bind to specific regions of promoter Provide a substrate for RNA polymerase to bind beginning transcription Forms transcription initiation complex

Elongation RNA molecule is built Primer not needed 5’ to 3’ direction RNA polymerase Primer not needed 5’ to 3’ direction Template strand is copied 3’ to 5’ DNA Coding strand DNA strand that is not copied Produces mRNA Messenger RNA DNA double helix reforms

Termination RNA polymerase recognizes a termination sequence – AAAAAAA (polyadenylation) Nuclear proteins bind to string of UUUUUU on RNA mRNA molecule releases from template strand

Post-Transcriptional Modifications Pre-mRNA undergoes modifications before it leaves the nucleus Poly(A) tail Poly-A polymerase Protects from RNA digesting enzymes in cytosol 5’ cap 7 G’s Initial attachment site for mRNA’s to ribosomes Removal of introns

Splicing the pre-mRNA DNA comprised of Spliceosome Exons Introns sequence of DNA or RNA that codes for a gene Introns non-coding sequence of DNA or RNA Being researched for code responsible for different splicing arrangements Spliceosome Enzyme that removes introns from mRNA

Splicing Process Spliceosome contains a handful of small ribonucleoproteins snRNP’s (snurps) snRNP’s bind to specific regions on introns

Alternative Splicing Increases number and variety of proteins encoded by a single gene ~25,000 genes produce ~100,000 proteins

Translation Takes place in cytoplasm mRNA to protein Ribosomes read codons tRNA assists ribosome to assemble amino acids into polypeptide chain

tRNA Contains Are there 61 tRNA’s to read 61 codons? triplet anticodon amino acid attachment site Are there 61 tRNA’s to read 61 codons?

tRNA: Wobble Hypothesis First two nucleotides of codon for a specific AA is always precise Flexibility with third nucleotide Aminoacylation process of adding an AA to a tRNA Forming aminoacyl-tRNA molecule Catalyzed by 20 different aminoacyl-tRNA synthetase enzymes

Ribosomes Translate mRNA chains into amino acids Made up of two different sized parts Ribosomal subunits (rRNA) Ribosomes bring together mRNA with aminoacyl-tRNAs Three sites A site - aminoacyl P site – peptidyl E site - exit

Translation process Three stages Initiation Elongation Termination 1 Codon recognition Amino acid Anticodon A site P site Polypeptide 2 Peptide bond formation 3 Translocation New peptide bond mRNA movement mRNA Stop codon Three stages Initiation Elongation Termination

Initiation Reading frame is established to correctly read codons Ribosomal subunits associate with mRNA Met-tRNA (methionine) Forms complex with ribosomal subunits Complex binds to 5’cap and scans for start codon (AUG) (scanning) Large ribosomal subunit binds to complete ribosome Met-tRNA is in P-site Reading frame is established to correctly read codons

Elongation Amino acids are added to grow a polypeptide chain A, P, and E sites operate 4 Steps

Termination A site arrives at a stop codon on mRNA UAA, UAG, UGA Protein release factor binds to A site releasing polypeptide chain Ribosomal subunits, tRNA release and detach from mRNA

polysome a b What molecules are present in this photo? Red object = ?

Review What is a gene? Where is it located? What is the main function of a gene? Do we need our genes “on” all the time? How do we turn genes “on” or “off”?